1. Field of the Invention
Illustrative embodiments of the present invention relate to an image forming apparatus, and more specifically, to an image forming apparatus having a recording head that ejects droplets.
2. Description of the Background
Image forming apparatuses are used as printers, facsimile machines, copiers, plotters, or multi-functional peripherals having two or more of the foregoing capabilities. As one type of image forming apparatus employing a liquid-ejection recording method, an inkjet recording apparatus is known that ejects liquid droplets from a recording head onto a recording medium to form a desired image (hereinafter “image formation” is used as a synonym for “image recording” and “image printing”).
Such inkjet-type image forming apparatuses fall into two main types: a serial-type image forming apparatus that forms an image by ejecting droplets from the recording head while moving the recording head in a main scan direction, and a line-head-type image forming apparatus that forms an image by ejecting droplets from a linear-shaped recording head held stationary in the image forming apparatus.
As for the recording heads (droplet ejection heads) used in these inkjet-type image forming apparatuses, several different types are known. One example is a piezoelectric recording head that ejects liquid droplets by displacing a diaphragm using a piezoelectric actuator or the like. Specifically, when the piezoelectric actuator displaces the diaphragm, the volume of a chamber containing the liquid is changed. As a result, the internal pressure of the chamber is increased, so that droplets are ejected from the head. Another example is a thermal recording head that ejects droplets by increasing the internal pressure of the chamber using a heater. This increase is accomplished, for example, using a heater located in the chamber that is heated by an electric current to generate bubbles in the chamber. As a result, the internal pressure of the chamber is increased, so that droplets are ejected from the head.
For such a liquid-ejection type image forming apparatus, there is demand for enhancing throughput, i.e., speed of image formation. For example, one liquid (in this case ink) supply method is proposed in which ink is supplied from a high-capacity ink cartridge (main tank) mounted in the image forming apparatus to a sub tank (also referred to as a head tank or buffer tank) mounted in an upper portion of the recording head through a tube. Such a tube supply method allows the weight and size of a carriage of the recording head to be reduced and enables downsizing of the structure, driving system, and image forming apparatus as a whole.
In this regard, in the tube supply method described above, ink is supplied from the ink cartridge to the recording head and consumed at the recording head during image formation. If, for example, a flexible thin tube is used, a relatively large fluid resistance arises when ink passes through the tube. Consequently, ink may not be supplied in time for ink ejection, thus causing ejection failure. In particular, as the size of the image forming apparatus increases, the length of the tube also increases, thus causing a larger resistance to ink passing through the tube. Alternatively, when high speed printing is performed or high viscosity ink is employed, such fluid resistance of the tube is increased, thus causing ink supply shortage.
Hence, one conventional technique is proposed in which ink in the ink cartridge is maintained in a pressurized state and a differential-pressure regulation valve is provided at an upstream side of the recording head in a direction in which ink is supplied (hereinafter, “ink supply direction”). In such a configuration, when negative pressure within the sub tank is greater than a predetermined pressure value, ink is supplied to the recording head.
However, for the conventional technique described above, although the above-described ink supply shortage is prevented, the mechanism for controlling negative pressure is complicated and a high level of sealing performance is required for a negative-pressure conjunction valve. Further, as constant pressurization is employed, a high level of air sealing is required for all connecting portions of the ink supply paths. Accordingly, a failure in any part of the sealing of the ink supply system might cause the ink to blow out.
In another conventional technique, a negative-pressure chamber maintained in a negatively pressurized state using a spring is provided at an upstream side of the recording head. In this configuration, ink supply pressure is actively controlled by feeding ink to the negative-pressure chamber using a pump. In still another conventional technique, the ink supply pressure is actively controlled using a pump without such a negative-pressure room.
In the above-described two techniques, when the ink supply pressure is actively controlled, the amount of ink fed using the pump is accurately controlled in response to the consumption amount of ink or the like. Further, when the above-described techniques are applied to an image forming apparatus using different color inks, the pump is separately controlled for each of the respective color inks. Such a configuration may require a complex control system and an increased size of the image forming apparatus.
One method of obtaining a negative pressure with a simple configuration is proposed in which an ink cartridge to the atmosphere is connected to a recording head through a tube and the ink cartridge is located at a position lower than the recording head to obtain a negative pressure using a difference in fluid level between fluid heads.
Such a fluid-level difference method can provide stable negative pressure using a very simple configuration as compared to the method of constantly applying pressure using a negative-pressure conjunction valve or the method of feeding ink using a negative-pressure chamber and a pump. However, in the fluid-level difference method, the above-described large tube resistance may cause pressure loss.
One conventional technique proposed to prevent such pressure loss in the ink supply system obtains a negative pressure using the fluid-level difference method, this time with a pump that is provided on a tube connecting the recording head to the ink cartridge. Further, a bypass is provided to connect an upstream side and a downstream side of the pump, and a valve is provided on the bypass. The degree of opening of the valve on the bypass is adjusted in response to printing process to maintain a desired pressure.
However, when the above-described conventional technique is applied to an image forming apparatus using different color inks, the pump must be separately controlled for respective color inks, resulting in an increased size of the image forming apparatus.